1. Field of the Invention
The present invention relates to a belt unit used in an image forming apparatus such as a copying machine, a printer, and a FAX machine employing an electrophotographic method or an electrostatic recording method, and to an image forming apparatus including the belt unit.
2. Description of the Related Art
In conventional image forming apparatuses employing an electrophotographic method and the like, an endless belt stretched between a plurality of support rollers, has been used as an intermediate transfer member, a recording material bearing member, or the like. A toner image is transferred from a photosensitive member onto the intermediate transfer member. The recording material bearing member carries and conveys a recording material onto which the toner image is transferred from the photosensitive member. Such a belt has a problem of belt deviation where the drivingly rotated belt is deviated in a direction toward any one of the end portions in a width-wise direction depending on an accuracy of the outer diameters of the rollers, an accuracy of alignment between the rollers, or the like.
Japanese Patent Application Laid-Open No. H9-169449 discusses a configuration of detecting the belt deviation with a sensor and controlling an inclination of a steering roller with an actuator, as a unit for correcting the belt deviation. Japanese Patent Application Laid-Open No. 2001-146335 discusses a configuration in which belt deviation restricting portion members that engage with a rib formed on a back surface of a belt are disposed on both end portions of a support roller. Japanese Translation of PCT Application No. 2001-520611 discusses a method of automatically aligning a belt with a steering roller based on friction force balance (hereinafter, referred to as “belt auto alignment method”), as a method of controlling the belt deviation easily at low cost and with a small number of components.
The configuration in Japanese Translation of PCT Application No. 2001-520611 includes a steering mechanism as illustrated in FIG. 9 of the present application. More specifically, the steering mechanism includes a steering roller 90 that can rotate by being driven by the rotation of a belt and both end members 91 that are disposed at both end portions of the steering roller 90 and cannot be driven by the rotation of the belt. The steering roller 90 is supported by a supporting base 92 that can rotate in a direction indicated by an arrow S about a steering shaft 93 disposed in a center portion. The supporting base 92 is biased in a direction indicated by an arrow PT by a tension applying unit 95 that is compressed by a pressure release cam 96. As a result, an outer circumference surface of the steering roller 90 applies a tension to an inner circumference surface of the belt (not illustrated).
The belt auto alignment method can achieve a simpler configuration than the method of controlling the inclination of the steering roller with the sensor and the actuator, and can achieve smaller degradation such as wearing of components than the method in which the rib of the belt comes into contact with the belt deviation restricting member. All things considered, the belt auto alignment method advantageously facilitates an attempt to reduce an apparatus cost.
When the belt unit is attached to or detached from the apparatus main body of the image forming apparatus, the tension applied to the belt might be reduced. The steering mechanism in the belt auto alignment method is inclined by means of the movement force of the belt. Thus, the steering roller is likely to incline when the tension applied to the belt is reduced. When the belt unit is attached to or detached from the apparatus main body in this state, the steering mechanism might come into contact with another member in the apparatus main body such as a photosensitive member, and thus the other member might be damaged by abrasion or the like.
In the steering mechanism in Japanese Translation of PCT Application No. 2001-520611, springs 98 serving as a rotation stopping unit for the steering mechanism are disposed at both end portions of a steering roller 90 in an axial direction. In this configuration, the tension applied to the belt might be relatively low not only for attaching or detaching the belt unit but also due to settings for image forming or for achieving a configuration of separating a roller, which comes into contact with the photosensitive member with a belt interposed in between, from the photosensitive member. In such a case, with the configuration in Japanese Translation of PCT Application No. 2001-520611, the force of the springs 98 might be relatively too strong and thus the belt auto alignment based on friction force balance might become ineffective. On the other hand, when the force of the springs 98 is set to be low, the steering mechanism is likely to incline. Thus, as in the case described above, when the belt unit is attached to or detached from the apparatus main body, the steering roller might come into contact with the photosensitive member and the like in the apparatus main body and thus the photosensitive member might be damaged by abrasion or the like.